Preparation Of Nitrogen-Doped Porous Carbon Cathode And Their Application For Refractory Organic Pollutants Treatment By Electro-Fenton

The electro-fenton?EF?technology is to continuously produce Fenton reagent H₂O₂ by electrochemical utilization of O₂ or air and react with Fe²⁺/Fe³⁺cycle to convert highly active hydroxyl radicals?·OH?.It combines the advantages of electrochemistry and Fenton reaction,and it is a high-level electrochemical advanced oxidation?EAOPs?organic pollution water treatment technology in recent years.This work utilizes nitrogen-doped porous carbon?NPC?carbonized by zeolitic imidazolate framework-8?ZIF-8?nanocrystals as carbon precursor,polytetrafluoroethylene?PTFE?as adhesive,graphite felt?GF?,carbon black?CB?and steel wire mesh,and to fabricate a new modified graphite felt of NPC/GF as carbonaceous cathode,a novel rolled NPC/CB carbonaceous gas-diffusion cathode,a jet-flow reactor NPC/CB/GF special cathode.We have studied the mechanism and properties of the main influencing factors in the process of electrocatalytic production of H2O2 and EF degradation of organic pollutants.The main research contents are as follows:1)The study develops a new modified GF as carbonaceous cathode for EF application loaded with NPC.At initial pH 7,the highest generation rate of H₂O₂ was 0.74 mg h^-11 cm^-22 by applying 12.5 mA cm^-22 by modified cathode,but in the same condition,the GF only had0.067 mg h^-11 cm^-2.The production efficiency incressed 10 times.The highest CE of NPC/GF electrode was 10.04%,and the lowest EEC was 216 kWh·kg^-1.After 5 cycles,the stability of NPC/GF electrode was maintained.Additionally,phenol(50 mg L^-1)could be largely removed by NPC modified cathode,the mineralization ratio and TOC reached 100%and82.61%at 120 min of optimization condition,respectively.The results showed that the removal rate of Rhodamine B was 100%in 40 min with 0.4 mM FeSO₄.The presence of·OH in the degradation process was investigated by ESR,which proved the effectiveness of the new electrode GF-4 in phenol degradation.The materials were characterized by scanning electron microscopy?SEM?,X-ray photoelectron spectroscopy?XPS?,X-ray diffraction?XRD?and linear sweep voltammetry?LSV?.The results demonstrated that a homogenous NPC covered the carbon-based material GF.The existing graphitic-N and sp² carbon of NPC promoted the electron transfer between carbon surface and oxygen molecules,as well as accelerated the oxygen reduction reaction?ORR?and the modified graphite felt had much higher electrocatalytic activity.In this work,several manufacturing parameters like the current,pH and load of NPC were optimized.The optimized design could improve the efficiency of new cathode with in situ electro-chemical production of H₂O₂ and significantly offer a potential material for degradation of organic pollutants.2)Subsequently,we utilizes NPC and carbon black?CB?to fabricate a novel rolled carbonaceous gas-diffusion cathode,which greatly improves the production of H₂O₂ to 7.27mg h^-1cm^-2by optimization in near neutral pH condition.When the cathode current density was 28 mA·cm^-22 and aeration rate was 0.6 L·min^-1,the cumulative concentration reached726.92 mg·L^-1in 2 h.Under the same conditions,the cumulative concentration of H₂O₂ in NPC/CB-?0%?was only 379.83 mg·L^-11 in 2 h.Meanwhile,the highest CE and lowest EEC of NPC/CB-?40%?electrode are 45.50%and 47.82 kWh·kg^-1respectively.Accordingly,it remained stable after 10 cycles.These cathodes were characterized by BET,SEM,LSV and electrochemical impedance spectroscopy?EIS?respectively,investigated for the H2O2generation of the ORR.Further application of NPC/CB cathode in heterogeneous electro-Fenton?EF?experiments with synthetic pyrite?FeS₂?as catalyst,metal sulfides?MoS₂,WS₂?as co-catalysts,the removal efficiency of the tetracycline(50 mg L^-1)could reach 100%and the TOC removal efficiency of counterpart could reach 81.3%,respectively.In this process,when the optimal amount of FeS2 was 0.75 g,the degradation was completed in 40min,and the adaptive pH value was maintained at 2.96-4.00 in 13-58 min,and the concentration of Fe²⁺/Fe³⁺reached 0.55/0.51 mmol·L^-11 in 120 min.Accordingly,FeS₂ was tested repeatedly and showed good adaptability in heterogeneous EF.In addition,under the optimal FeS₂ dosage of 0.75 g,0.08 g and 0.12 g of MoS₂ and WS₂ cocatalyst were added,tetracycline was completely degraded in 19 min and 29 min,and Fe²⁺/Fe³⁺concentration reached 0.22/0.05 mmol·L^-11 at 60 min,but only 0.14/0.09 mmol·L^-11 for FeS₂ alone.In addition,tannery wastewater was pretreated under the same experimental conditions.When cod,TN and NH₃-N of raw water are about 1578 mg·L^-1,66 mg·L^-1and 150 mg·L^-1respectively,and the initial pH value is not adjusted,the mineralization rate of COD is about69.62%,the degradation rate of TN is 72.83%,and the degradation rate of NH₃-N is 14.24%.Accordingly,the cost of NPC/CB-?40%?electrode current consumption was calculated,and the power consumption cost of 1 ton tannery wastewater treatment was 17.575 yuan.Because of the FeS₂ produces H⁺and Fe²⁺in the presence of H₂O₂,which provides pH value and catalytic ion that suitable for EF.Meanwhile,the metal sulfides can accelerate the conversion of Fe³⁺to Fe²⁺by exposing reducible metal active sites on the surface of the catalysts,thus drastically increased the efficiency of H2O2 decomposition and pollutant degradation,also optimized the disadvantage of little Fe²⁺leaching from FeS₂ in heterogeneous EF.Further tests of this system indicated its potential for the treatment of tannery effluents.3)Furthermore,a new jet-flow reactor was applied in EF,and the effects of ground current density,water flow rate,Fe²⁺concentration,MoS₂,initial pH value and UV irradiation on the removal rate of Rhodamine B were studied.The special electrode NPC/CB/GF was prepared.It was found that the highest accumulation concentration of H₂O₂ was 792.40mg·L^-1and the generation rate of H₂O₂ was 7.92 mg h^-1cm^-2within 2 h under current density24 mA cm^-2,pH 7 and water flow 50 L·min^-1.At the same time,the degradation rate of Rhodamine B reached 100%in 10 min under the system of Fe SO4 and MoS2photo-electric-fenton,and the EEO reached 72 kWh m^-33 order^-11 in 5 min.Compared with the traditional aeration,the new jet reactor makes the gas-liquid mixture more fully.The process could improve the removal efficiency of pollutants in organic effluents,and provided an effective and feasible scheme for promoting the improvement of the EF system and the development of practical application.